The present invention relates to a process for the simultaneous measurement of the geometrical characteristics of an optical fibre and in particular the optical core - mechanical sheath concentricity and the ellipticity of the optical core of an optical fibre, as well as to a device for performing this process.
As is known, an optical fibre comprises a core and an optical sheath. The core and sheath essentially differ by their optical - refractive index. The evolution of this optical refractive index takes place in accordance with a predetermined profile. After drawing, the optical fibre usually comprises its two regions, a support tube and an external protective sheath placed around the latter during a subsequent manufacturing stage.
There are two types of fibre. Firstly, multimode fibres, which generally have an index gradient for which the core diameter is typically 50 micrometers and the external optical sheath diameter 70 micrometers. Monomode fibres, generally have an index jump, for which the core diameter is between 5 and 10 micrometers, whilst the external diameter of the optical sheath is approximately 40 micrometers. The external diameter of the support tube, which is usually made from pure silica, is approximately 125 micrometers and the external diameter of the protective sheath, e.g. of silicone, is approximately 300 micrometers.
The appearance of optical fibres made from silica/doped silica have made it possible to significantly reduce the optical transmission losses of optical fibres. It is at present possible to produce fibres having losses below 0.5 dB/km. The fibres must be very homogeneous to obtain such performance levels. In particular, to obtain a coupling between two fibres having low losses, it is necessary for the geometrical characteristics of these two fibres to be very close together. It is therefore necessary to check these characteristics very accurately and in particular the core-sheath concentricity and the core ellipticity. It is also necessary to carry out an adequate number of measurements to ensure the homogeneity of these geometrical characteristics along the fibre.
It is known to carry out these measurements by using a microscope having a reticule consisting of two concentric circles corresponding to the optical core and the mechanical sheath of an ideal optical fibre, i.e. with the optical core and mechanical sheath perfectly cylindrical and concentric with respect to one another, the core of said optical fibre being illuminated by a beam of white light. In this case, the protective envelope has been removed before the measurement. However, this measurement only makes it possible to carry out a classification or sorting with respect to limiting ellipticity and eccentricity values and gives no order of magnitude for these. Thus, this process is purely manual and does not make it possible to carry out precise measurements of the characteristics to be measured and only enables a sorting to be made.